3-Hydroxyflavones (3-HF) are important fluorescent sensors due to their excited state intramolecular proton transfer (ESIPT) property originated from their normal (N*) and phototautomer forms (T*) (T. Ozturk, A. S. Klymchenko, A. Capan, S. Oncul, S. Cikrikci, S. Taskiran, B. Tasan, F. B. Kaynak, S. Ozbey, A. P. Demchenko, Tetrahedron, 2007, 63, 10290). This property provides 3-HF with well separated two emission bands on fluorescent spectroscopy, resulting from their excited normal and tautomeric forms, intensities of which are sensitive to their environment, including polarity and hydrogen bonding perturbations in the surrounding molecules (P. Chou, D. McMorrow, T. J. Aartsma, M. J. Kasha, Phys. Chem. 1984, 88, 4596). The changes in the peak intensities and in their corresponding ratios (N*/T*) have been well observed in proteins (A. Sytnik, D. Gormin, M. Kasha, Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11968), micelles (M. Sarkar, J. G. Ray, P. K. Sengupta, Spectrochim. Acta Part, A 1996, 52, 275) and polymers (J. R. Dharia, K. F. Johnson, J. B. Schlenoff, Macromolecules 1994, 27, 5167).
As the presence of electron donating groups at C-3 of 3-HFs improves their fluorescence strength, sensor researches related with 3-HFs have particularly been devoted to such materials. Ratiometric changes between the two peaks, depending on the corresponding changes at the surrounding environment of 3-HFs have created various applications, among which are sensing ions and moisture, recognition of organized systems like micelles and phospholipids (A. P. Demchenko, K.-C. Tang, P.-T. Chou, Chem, Soc. Rev, 2013, 42, 1379).